docs: def check_aux_coordinates(self, ds):
          '''
          Chapter 5 paragraph 3
  
          The dimensions of an auxiliary coordinate variable must be a subset of
          the dimensions of the variable with which the coordinate is associated,
          with two exceptions. First, string-valued coordinates (Section 6.1,
          "Labels") have a dimension for maximum string length. Second, in the
          ragged array representations of data (Chapter 9, Discrete Sampling
          Geometries), special methods are needed to connect the data and
          coordinates.
  
          :param netCDF4.Dataset ds: An open netCDF dataset
          :rtype: list
          :return: List of results
          '''
  
          ret_val = []
          geophysical_variables = self._find_geophysical_vars(ds)
          for name in geophysical_variables:
              variable = ds.variables[name]
              coordinates = getattr(variable, 'coordinates', None)
              # We use a set so we can assert
              dim_set = set(variable.dimensions)
              # No auxiliary coordinates, no check
              if not isinstance(coordinates, basestring) or coordinates == '':
                  continue
  
              valid_aux_coords = TestCtx(BaseCheck.HIGH,
                                        self.section_titles["5"])
  
              for aux_coord in coordinates.split():
                  valid_aux_coords.assert_true(aux_coord in ds.variables,
                                               "{}'s auxiliary coordinate specified by the coordinates attribute, {}, "
                                               "is not a variable in this dataset"
                                               "".format(name, aux_coord))
                  if aux_coord not in ds.variables:
                      continue
  
                  # §6.1 Allows for "labels" to be referenced as coordinates
                  if ds.variables[aux_coord].dtype.char == 'S':
                      continue
  
                  aux_coord_dims = set(ds.variables[aux_coord].dimensions)
                  valid_aux_coords.assert_true(aux_coord_dims.issubset(dim_set),
                                               "dimensions for auxiliary coordinate variable {} ({}) "
                                               "are not a subset of dimensions for variable {} ({})"
                                               "".format(aux_coord,
                                                         ', '.join(aux_coord_dims),
                                                         name,
                                                         ', '.join(dim_set)))
              ret_val.append(valid_aux_coords.to_result())
          return ret_val